August 17, 1888.] 



SCIENCE. 



n 



mend the reading of these remarkable original memoirs to any 

 physicist who knows them only at second-hand. 



One more significant lesson remains, in the effect of this on the 

 minds of his contemporaries. Herschel's observation is to us 

 -almost a demonstration of the identity of radiant heat and light ; 

 but now, though the nineteenth century is opening, it is with the 

 •doctrine still in the minds of most physicists, and perhaps of all 

 chemists, that heat is occasioned by a certain material fluid. 

 Phlogiston is by this time dead or dying, but caloric is very much 

 alive, and never more perniciously active than now, when, for in- 

 stance, years after Herschel's observation, we find this cited as 

 ■" demonstrating the e.\istence of caloric," which was, it seems, 

 the way it looked to a contemporary. 



In the year 1S04 appeared what should be a very notable book in 

 the history of our subject, written by Sir John Leslie, whose name 

 survives perhaps in the minds of many students chiefly in connec- 

 tion with the 'cube,' which is still called after him. 



Leslie, however, ought to be remembered as a man of original 

 genius, worthy to be mentioned with Herschel and Melloni ; and 

 his, too, is one of the books which the student may be recommended 

 to read, at least in part, in the original ; not so much for the writer's 

 instructive experiments (which will be found in our text-books) as 

 for his most instructive mistakes, which the text-book will probably 

 not mention. 



He began by introducing the use of the simple instrument which 

 bears his name, and a new and more delicate heat-measure (the 

 differential thermometer); and with these, and concave reflectors of 

 glass and metal, he commenced experiments in radiant heat, than 

 which, he tells us, no part of physical science then appeared so 

 dark, so dubious, and so neglected. It is interesting, and it marks 

 the degree of neglect he alludes to, that his first discovery was that 

 ■different substances have different radiating and absorbing powers. 

 It gives us a vivid idea of the density of previous ignorance, that it 

 was left to the present century to demonstrate this elementary fact, 

 and that Leslie, in view of such discoveries, says, " I was trans- 

 ported at the prospect of a new world emerging to view." 



Next he shows that the radiating and absorbing powers are pro- 

 portional, next that cold as well as heat seems to be radiated, and 

 next undertakes to see whether this radiant heat has any affinity to 

 light. 



He then experiments in the ability of radiant heat to pass through 

 a transparent glass, which transmits light freely, and thinks he 

 finds that none does pass. Radiant heat with him seems to mean 

 heat from non-luminous sources ; and the ability or non-ability of 

 this to pass through glass is to Leslie and his successors a most 

 crucial test, and its failure to do so a proof that this heat is not 

 affiliated to light. 



Let us pause a moment here to reflect that we are apt to uncon- 

 sciously assume, while judging from our own present standpoint 

 where past error is so plain, that the false conclusion can only be 

 chosen by an able, earnest, conscientious seeker, after a sort of strug- 

 gle. Not so. Such a man is found welcoming the false with rap- 

 ture as very truth herself. 



" What, then," says Leslie, " is this calorific and frigorific fluid 

 after which we are inquiring .' It is not light, it has no relation to 

 ether, it bears no analog)- to the fluids, real or imaginary-, of mag- 

 netism and electricity. But why have recourse to invisible agents .' 

 Quod pctis, hie est. It is merely the ambient AIR." 



The capitals are Leslie's own, but ere we smile with superior 

 knowledge let us put ourselves in his place, and then we may com- 

 prehend the exultation with which he announces the identity of 

 radiant heat and common air, for he feels that he is beginning a 

 daring revolt against the orthodox doctrine of caloric ; and so he 

 is. 



The first five years of this century are notable in the historj- of 

 radiant energy, not only for the work of Leslie, and for the observa- 

 tion by Wollaston. Ritter, and others, of the so-called 'chemical' 

 rays beyond the violet, but for the appearance of Young's papers, 

 re-establishing the undulatory theory, which he indeed considered 

 in regard to light, but which was obviously destined to affect most 

 powerfully the theory of radiant energy in general. 



We are now in the year 1804, or over a century and a quarter 

 since the corpuscular theory was emitted, and during that time it 



has gradually grown to be an article of faith in a sort of scientific 

 church, where Newton has come to be looked on as an infallible 

 head, and his views as dogmas, about which no doubt is to be tol- 

 erated ; but if we could go back to Cambridge in the year 1668, 

 v/hen the obscure young student, in no way conscious of his future 

 pontificate, takes his degree ("standing twenty-third on the list of 

 graduates), we should probably find that he had already elaborated 

 certain novel ideas about the undulatory theory of light, which he 

 at any rate promulgates a few years later, and afterward, pressed 

 with many difficulties, altered, as we now know, to an emissive 

 one. 



Probably, if we could have heard his own statement then, he 

 would have told how sorely tried he was between these two opin- 

 ions, and, while explaining to us how the wavering balance came 

 to lean as it did, would have admitted, with the modesty proper to 

 such a man, that there was a great deal to be said on either side. 

 We may, at any rate, be sure that it would not be from the lips of 

 Newton himself that we should have had this announced as a be- 

 lief which was to be part of the rule of faith to any man of science. 



But observe how, if science and theology look askance at each 

 other, it is still true that some scientific men and some theologians 

 have, at any rate, more in common than either is ready to admit ; 

 for at the beginning of this century Newton's followers, far less 

 tolerant than their master, have made out of this modest man a 

 scientific pontiff, and out of his diffident opinions a positive dogma, 

 till, as years go on. he comes to be cited as so infallible that a 

 questioning of these opinions is an offence deserving excommunica- 

 tion. 



This has grown to be the state of things in 1804, when Young, a 

 man possessing something of Newton's own greatness, ventures to 

 put forward some considerations to show that the undulatory theory- 

 may be the true one, after all. But the prevalent and orthodox 

 scientific faith was still that of the material nature of light ; the 

 undulatory hypothesis was a heresy, and Young a heretic. If his 

 great researches had been reviewed by a physicist or a brother 

 worker, who had himself trodden the difiicult path of discovery, he 

 might have been treated at least intelligently; but then, as always, 

 the camp-followers, who had never been at the front, shouted from 

 a safe position in the rear to the man in the dust of the fight, that 

 he was not proceeding according to the approved rules of tactics ; 

 then, as always, these men stood between the public and the inves- 

 tigator, and distributed praise or blame. 



If you wish to hear how the scientific heretic should be rebuked 

 for his folly, listen to one who never made an observation, but, hav- 

 ing a smattering of every thing books could teach about every 

 branch of knowledge, was judged by himself and by the public to 

 be the fittest interpreter to it, of the physical science of his day. I 

 mean Henry Brougham, the future lord-chancellor of England, the 

 universal critic, of whom it was observed, that, " if he hacf but 

 known a little law, he would have known a little of every thing." 

 He uses the then all-powerful Edinburgh Revieiu forhis pulpit, and 

 notices Young's great memoir as follows : — 



" This paper contains nothing which deserves the name either of 

 experiment or discovery ; and it is, in fact, destitute of every species 

 of merit. . . . The paper which stands first is another lecture, con- 

 taining more fancies, more blunders, more unfounded hypotheses, 

 more gratuitous fictions . . . and all from the fertile yet fruitless 

 brain of the eternal Dr. Young. In our second number we exposed 

 the absurdity of this writer's ' law of interference,' as it pleases him 

 to call one of the most incomprehensible suppositions that we re- 

 rnember to have met with in the history of human hypotheses." 



There are whole pages of it, but this is enough ; and I cite this 

 passage among many such at command, not only as an example of 

 the way the undulatory theory was treated at the beginning of this 

 century in the first critical journal of Europe, but as another ex- 

 ample of the general fact that the same thing may appear intrinsi- 

 cally absurd, or intrinsically reasonable, according to the year of 

 grace in which we hear of it. The great majority, even of students 

 of science, must take their opinions ready-made as to science in 

 general ; each knowing, so far as he can be said to know any thing 

 at first-hand, only that little corner which research has made spe- 

 cially his own. 



The moral we can all draw, I think, for ourselves. 



